A ball screw is provided with: a nut having an inner circumferential surface on which a spiral groove is formed; a threaded shaft having an outer circumferential surface on which a spiral groove is formed; balls loaded in a raceway between the spiral groove of the nut and that of the threaded shaft; and a ball return passage for returning the balls from an end point of the raceway to a start point thereof. The ball screw is a device such that the balls are rolling in the raceway to make the nut relatively move with respect to the threaded shaft.
Such ball screws are used in not only positioning devices of general industrial machinery but also electrical actuators mounted on vehicles such as automobiles, motorcycles, or vessels.
As to the ball return passage in the ball screw, there are a circulation tube type of the ball return passage and a deflector type thereof. In the case of the deflector type, a deflector having a concave constituting the ball return passage is fit in a through hole of the nut. In contrast, Patent Document 1 listed below describes that the concave (circulation groove) constituting the ball return passage is directly arranged on the inner circumferential surface of the nut blank by plastic working. The method for forming it will be described with reference to FIG. 19.
Firstly, prepared is a died provided with a cylindrical working head 30 having convexes 37 and 38 each having a letter S shape to correspond to the shape of a circulation groove. Then, a nut blank 1 is disposed on a table 200 with its axial direction set to the horizontal, the working head 30 is disposed inside the nut blank 1, the convexes 37 and 38 are faced upwardly, and a base end portion 30a and a front end portion 30b are secured. Next, in this state, pressure is applied onto a top member 20 of the die to press it down. The convexes 37 and 38 are pressed against an inner circumferential surface 11 of the nut blank 1 to plastically deform the inner circumferential surface 11 of the nut blank 1.